The Gameplay Engine. Both Horizon games were actually produced using the Decima Engine.
The Most Difficult Character To Write.
From the creative team to the engine being used and the way the original game inspired this follow-on, there are plenty of behind-the-scenes details that should help fans understand how this incredible release was really put together.
There may be some spoilers below surrounding the title and the future of the franchise!
Interestingly, the engine, which boasts its own physics system, artificial intelligence, and graphics package which has managed to evolve with next-gen consoles